Observations Concerning Culex pipiens as a Possible Carrier of St. Louis Encephalitis

Abstract

It has been shown by Casey and Broun that St Louis encephalitis cases appear to occur with higher incidence in those areas of the city and county which are adjacent to small streams and open ditches. 1 This suggests the possibility of a water breeding insect as a possible transmitting agent. Transmission of the equine types of encephalomyelitis by mosquitoes from animal to animal has been demonstrated by Kelser 2 , Simmons, Reynolds and Cornell 3 and Merrill, Lacaillade and Ten Broeck. 4

Webster, Clow and Bauer. 5 demonstrated the St. Louis encepha litis virus could be taken into the body of the Anoophles quadrimaculatus mosquito and retained for the duration of their lives. The virus containing mosquitoes, however, did not infect mice or monkeys by biting. Attempts were made subsequent to the 1933 epidemic to transmit encephalitis from human being to human being by the bite of the various species of mosquito without success. 6

Since Culex pipiens is the most common type of mosquito in the St. Louis area, we have studied the ability of this mosquito to become infected with the virus of St. Louis encephalitis.

Mice infected with St. Louis encephalitis by intraperitoneal injection of heavy doses of virus have been shown By Webster and his co-workers to have a considerable concentration of the virus in the circulating blood for a period of five hours after the injection. In our experiments 1 cc of a 1/10 dilution of virus containing brain was injected intraperitoneally. The mouse was then placed in a specially built biting cage where he was exposed to a number of mosquitoes for a period of 5 hours after dark in a quiet room.